The invention relates to a belt tensioner, especially a rotary tensioner, for a seat belt, comprising an elongate, preferably cylindrical pressure cylinder a piston movably supported in the pressure cylinder in the longitudinal direction adapted to be loaded by pressurized fluid and thus to be moved in a tensioning direction, a thrust means interacting with the piston and a stop provided on the inside of the pressure cylinder against which the piston may abut in the tensioning direction at an end position at which the piston seals the pressure cylinder.
From prior art belt tensioners comprising an inflator are known which include a pressure cylinder in which a piston movably supported in the longitudinal direction can be moved in the tensioning direction by fluid under pressure. A thrust means interacting with the belt reel, for example a toothed rack, is displaced by the piston, thus causing the belt reel to be rotated in a tensioning direction. On the inside of the pressure cylinder a stop is provided against which the piston can abut in the tensioning direction and thus cannot be moved out of the pressure cylinder. The stop and/or the piston are configured so that the pressure cylinder is sealed when the piston abuts against the stop. This reliably prevents the fluid under pressure, for example hot gas from an inflator, from escaping from the pressure cylinder.